The rise of mobile payments at coffee bars, food trucks and ride shares has made it harder for consumers to know when it's appropriate to tip and how much, according to Ted Rossman, industry analyst at CreditCards.com. "There's a whole big murky middle."

When presented with a variety of suggested tipping options, about 1 in 6 millennials said they regularly choose the lowest option, and nearly 1 in 5 gives no tip — more than any other age group.